This invention relates to a dielectric resonator for use, for example, in a filter device of high-frequency radio (wireless) equipment, and also to a method of producing such a dielectric resonator.
A conventional dielectric resonator designed to have a reduced length is shown in FIGS. 17 and 18. FIG. 17 is a perspective view of this dielectric resonator, and FIG. 18 is a cross-sectional view taken along the line XVIII--XVIII of FIG. 17. More specifically, a dielectric body 1 of a pillar-like or columnar shape has a through hole 4 extending from an upper surface 2 to a lower surface 3, and an outer peripheral surface of the body 1 has a stepped shape. An electrically-conductive film 5 is formed on the entire surface of the body 1 except for the upper surface 2, thus providing a dielectric resonator 6.
In order to obtain a predetermined resonance frequency with respect to a conventional dielectric resonator of the above coaxial type, it is necessary that the length of an electrically-conductive film on the outer peripheral surface of a dielectric body, as well as the length of the electirically-conductive film on the inner peripheral surface of a through hole formed through the body, should be greater than a predetermined length. For this reason, it has been difficult to shorten the length of the body.
To overcome this difficulty, there has been proposed the dielectric resonator of FIGS. 17 and 18 in which the outer peripheral surface of the body 1 is stepped so as to be increased in length, so that the length of the electrically-conductive film 5 on the outer peripheral surface of the body 1 can be increased. By doing so, the length of the electrically-conductive film 5 is not shortened despite the reduced length of the body 1. Thus, the length of the body 1 can be shortened.
The following requirements must be met in order to further shorten the length of the body 1 of the dielectric resonator shown in FIGS. 17 and 18.
First, referring to reference characters in FIG. 18, l1 represents the length of the lower portion of the body 1, and l2 represents the length of the upper portion of the body 1 (the upper and lower portions of the body 1 are separated from each other by the step portion on the outer peripheral surface of the body 1) also, lt represents the overall length of the body 1, and al represents the radius of the through hole 4. In addition, b1 represents the radius of the lower portion of the body 1, and b2 represents the radius of the upper portion of the body 1.
In order to further reduce the length lt of the body 1 of the dielectric resonator shown FIGS. 17 and 18, l1=l2 must be satisfied, and besides the impedance ratio K (K=1n(b2/a1)/1n(b1/a1)) must be reduced. However, in order to reduce the impedance ratio K, it is necessary either to increase a1 or to decrease b2, in which case the volume of the body 1 is reduced. This results in a problem that the selectivity of the no-load (Q) is lowered.